http://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment"The Pound-Rebka experiment is a well known experiment to test AlbertEinstein's theory of general relativity. It was proposed by RobertPound and his graduate student Glen A. Rebka Jr. in 1959, and was thelast of the classical tests of general relativity to be verified (inthe same year). It is a gravitational redshift experiment, whichmeasures the redshift of light moving in a gravitational field, or,equivalently, a test of the general relativity prediction that clocksshould run at different rates at different places in a gravitationalfield. It is considered to be the experiment that ushered in an era ofprecision tests of general relativity. (...) When the photon travelsthrough a gravitational field, its frequency and therefore its energywill change due to the gravitational redshift."

http://student.fizika.org/~jsisko/Knjige/Klasicna%20Mehanika/David%20Morin/CH13.PDFDavid Morin (p. 4): "This GR time-dilation effect was first measuredat Harvard by Pound and Rebka in 1960. They sent gamma rays up a 20mtower and measured the redshift (that is, the decrease in frequency)at the top. This was a notable feat indeed, considering that they wereable to measure a frequency shift of gh/c^2 (which is only a few partsin 10^15) to within 1% accuracy."

QUESTION: If, in a gravitational field, the speed of light variesexactly as the speed of cannonballs does, in accordance with Newton'semission theory of light, would Pound and Rebka have measured the same"frequency shift of gh/c^2"? It is easy to show that the answer is anunambiguous yes (Einsteinians occasionally admit that). How then canthe Pound-Rebka experiment be "the experiment that ushered in an eraof precision tests of general relativity"?